When distro-packaged content is in /bin and /sbin (and /lib, /usr etc), you can't easily have those 4 directories in a single image that you can atomically handle (upgrade, snapshot, mount, share, etc) and you must have them together on the /.
Ideally, and we are getting there, / should be tmpfs/rootfs, it's populated with the needed structure by the initramfs, the distro is mounted on /usr (readonly squashfs perhaps?), user data is /home, system data is /var, configuration is /etc. And the system should boot with empty /home /var and /etc ideally. Soon...
That's why Android has /system, and Linux has /usr now. (https://www.freedesktop.org/wiki/Software/systemd/TheCaseFor...).
Ideally, and we are getting there, / should be tmpfs/rootfs, it's populated with the needed structure by the initramfs, the distro is mounted on /usr (readonly squashfs perhaps?), user data is /home, system data is /var, configuration is /etc. And the system should boot with empty /home /var and /etc ideally. Soon...